Almost all computer systems use a display for visible output to a user. Image data is commonly supplied from the computer to the display in the form of an analog signal representing the brightness of the signal along successive lines of a scanning display raster. One commonly used standard for such video signals is the video graphics array (VGA) standard, together with extensions to higher resolutions. The basic VGA standard provided a screen image 640 pixels wide by 480 pixels high, with each row of pixels formed by a line of the scanning raster. A modern display may have a screen image, for example, 1920×1200 pixels. In this example, the screen refresh rate may be 60 frames per second, or 72,000 lines per second.
The VGA standard provides parallel analog signals for red, green, and blue (RGB) channels that can be fed without further processing of the data to an RGB display, for example, a cathode ray tube (CRT). Although digital displays connected by digital interfaces are becoming increasingly common, many CRT and other analog displays are still in use, and many recent computers and displays still support the analog VGA standard for compatibility.
Analog signals are susceptible to analog interference, and an interfering AC signal may cause a periodic modulation in the signal that appears as a fluctuation in the brightness of the image on the display. In the present example, an interfering signal with a frequency of a few hundred kHz (referred to as a “low frequency” signal in the context of computers where operating frequencies of MHz to GHz are normal) can produce a fluctuation repeating several times along the length of the line, that is to say, across the width of the display. This fluctuation in successive lines forms vertical or diagonal stripes over the display that can be unpleasant and distracting for the user. Sources of interference in that frequency range are common. For example, one standard type of switching power regulator found in many computers operates at 300 kHz.
Such “wavy” video has hitherto been detected and assessed primarily by human viewers looking at the display screen. However, the detection of waviness is then at best dependent on the visual acuity of the particular viewer on the particular occasion, and it has been found that the quality of assessment deteriorates markedly as the viewer continues looking at display screens.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the apparatus and methods as claimed.